This application claims the benefit of Japanese Patent Applications No. 2001-181967, No. 2002-032818, No. 2002-060742 and No. 2002-078481 which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an electric power steering apparatus for an automobile.
2. Related Background Art
Steering systems of automobiles widely use a so-called power steering apparatus for assisting a steering operation by use of an external power source. the power source for the power steering apparatus has hitherto involved the use of a vane hydraulic pump, and the engine is driven by this hydraulic pump in many cases. This type of power steering apparatus has, however, a large drive loss (on the order of several through ten horsepower (HP) at a maximum load) of the engine because of driving the hydraulic pump at all times and is therefore hard to apply to mini-sized motor vehicle having small displacements. Even the automobiles having comparatively large displacements, when installed, come to have such an inevitable demerit that their traveling fuel efficiencies become low enough not to be ignorable.
Such being the case, an electric power steering (which will hereinafter be abbreviated to EPS) involving the use of an electric motor as a power source has attracted the attention over the recent years for obviating those problems. The EPS apparatus exhibits characteristics, wherein there is no direct drive loss of the engine because of using a battery loaded into the car as a power source for the electric motor, a decrease in the traveling fuel efficiency is restrained as the electric motor is started only when assisting the steering operation, and in addition the electronic control can be extremely easily performed.
On the other hand, a steering gear for the automobile is high in rigidity and low in weight, and hence a rack-and-pinion steering gear is mainly employed at the present. Then, the EPS apparatuses for the rack-and-pinion steering gear include a column assist type in which the electric motor is disposed sideways of the column in order to drive a steering shaft and a pinion itself, and in addition a ball screw rack assist type in which a rack shaft is driven by an electrically-driven ball screw mechanism. In the ball screw rack assist type EPS apparatus (which will hereinafter be simply called the rack assist type EPS apparatus), the assist force does not act on a meshing surface between the pinion and the rack, and therefore a contact surface pressure between these two members, which is to be a factor for abrasion and deformation as well, is comparatively small.
According to the rack assist type EPS apparatus, a ball screw shaft male thread groove formed in a rack shaft engages with a female thread groove formed in a ball nut through a multiplicity of circulation balls (steel balls), and the ball nut is rotated by the electric motor, thereby moving the rack shaft in the axial directions. A power transmission method for transmitting the power from the electric motor to the ball nut may be a timing belt system disclosed in Japanese Utility Model Publication No.5-14939 (a first prior art) etc, however, a general method is a gear system disclosed in Japanese Utility Model Publication No.2-46455 (a second prior art) etc.
(1) In a rack assist type EPS apparatus, an electric motor is installed by using bolts to a steering gear case, and the installing mode thereof is classified into a one-point support type and a two-point support type. For example, according to the second prior art, the electric motor is fastened at its one end by bolts to a ball screw mechanism housing protruding from the steering gear case.
Further, according to the first prior art, the electric motor is fastened at its one end by bolts to the ball screw mechanism housing and further fastened at the other end likewise by bolts to the side surface of the steering gear case.
By the way, the rack assist type EPS apparatus adopts in many cases the electric motor that is small in its major diameter and comparatively large in its total length in terms of a space for installation for the convenience""s sake of being attached to the steering gear disposed at a lower portion of the car body. Therefore, if the electric motor is attached in the one-point support mode to the steering gear case, an inertial moment is large due to vibrations caused when traveling on a rough road etc, and an excessive bending stress etc acts on the steering gear case, with the result that the steering gear case might be deformed and damaged. Then, when scheming to increase a rigidity and a strength of the electric motor fitting portion (the ball screw mechanism housing etc) of the steering gear case in order to prevent the deformation, damage and so on, increases in frame size and in weight are inevitable from a necessity of obtaining an increased wall thickness of this fitting portion.
While on the other hand, in the case of fitting the electric motor to the steering gear case in the two-point support mode, if the steering gear case gets flexural due to the vibrations caused when traveling and if the electric motor undergoes a thermal expansion due to frequent actuations thereof, a distance between the supporting or fitting portions to the steering gear case is not coincident with a total length of the electric motor. As a result, a compression stress or a tensile stress acts on the electric motor and on the steering gear case, with the result that the electric motor might fall into a failure and the steering gear case might be deformed and damaged. Further, in the case of taking a structure in which the steering gear case and the electric motor are disposed horizontally in a way that supports the side surface of the electric motor with a stay in order to ensure a minimum road clearance in the electric power steering apparatus, dusts and mud accumulate in between the steering gear case and the electric motor and on an upper surface of the shelved stay, and then contain a water content of rain, resulting in rusting of each of the constructive members as the case may be.
(2) A rack assist type EPS apparatus, of which an electric motor is disposed on an axis different from the ball nut, uses the electric motor that is larger in output and in size than a coaxially-disposed electric motor, and hence it is difficult to ensure a heat-radiation air space in periphery of the electric motor. Consequently, a temperature of the electric motor receiving the heat from the engine, an exhaust pipe etc, rises depending on where the electric motor is installed, and a predetermined output might not be obtained.
It is desirable for preventing this rise in temperature that the electric motor be installed as away from the heat source as possible. To be specific, since the engine and the exhaust pipe are disposed comparatively upward, a lower portion of the engine room is highly available for installing the electric motor. In the case of installing the electric motor in the vicinity of the steering gear case, however, it is required that the minimum ground clearance should not be reduced to the greatest possible degree in order to avoid interference with an obstacle when traveling on the rough road.
(3) In a rack assist type EPS apparatus with an electric motor disposed on an axis different from the ball nut, it follows that the electric motor is positioned in the lower portion of the car body, which is comparatively close to the road surface. It is therefore inevitable that the water is splashed over the electric motor and an electric harness thereof when traveling in the rain and passing through the puddles and that the pebbles rebounding from the tires when traveling on the rough road impinge thereon.
As known well, the output (torque generated) of the electric motor changes depending on its temperature, and hence, when the electric motor is abruptly cooled by the splashed water, it follows that the steering assist force of the EPS apparatus changes abruptly. This has been a factor for hindering smooth steering by the driver and making a steering feeling unagreeable. Further, the impingement of the peddles upon the electric motor, the electric harness, a connector and so forth, has been a factor for rusting due to exfoliation of coating and plating on the motor body and for causing a disconnection and a short-circuit of the electric circuit. On the other hand, a housing generally composed of an aluminum alloy etc accommodates a gear train and a belt defined as a power transmission mechanism between the electric motor and the power assist mechanism, but might be cracked by the impingement of the rebounding peddles, with the result that a lubricant might leak out and the muddy water might permeate, and so on.
It is an object of the first invention, which as devised to obviate the problem (1) given above, to provide a rack assist type electric power steering apparatus capable of improving a method of supporting an electric motor.
According to the first invention, an electric power steering apparatus comprises an electric motor having a motor housing, a ball screw mechanism connected to the electric motor through a power transmission mechanism and used for driving a rack shaft, and a steering gear case encasing the rack shaft and the ball screw mechanism, wherein the electric motor is supported by at least two supporting portions with respect to the steering gear case, and is supported by at lest one of the two supporting portions through an elastic member.
According to the first invention, the electric motor is supported by the two or more support portions, and therefore an excessive bending stress etc does not act on the steering gear case even when traveling on a bad road etc. Further, the electric motor is elastically supported by at least one support portion, and hence, even when the steering gear case get flexural and the electric motor thermally expands, almost no compression stress nor tensile stress acts on the both.
Further, in the electric power steering apparatus according to the first invention, preferably a small-diameter portion may be formed at an end portion of the motor housing, and one portion of the supporting portions may be used for supporting the small-diameter portion. With this contrivance, a support bracket etc can be downsized, whereby both of a weight of the apparatus and a manufacturing cost can be reduced.
In the electric power steering apparatus according to the first invention, preferably a vertical elastic coefficient of the elastic member may change stepwise. With this scheme, for instance, minute vibrations of the electric motor are absorbed by the elastic member in an area exhibiting a low vertical elastic coefficient, while a comparatively large relative shift occurred between the steering gear case and the electric motor is absorbed by the elastic member in an area exhibiting a comparatively high vertical elastic coefficient, thereby preventing bottoming and a metal touch as well.
In the electric power steering apparatus according to the first invention, preferably the elastic member may elastically support the motor housing in axial and radial directions. Owing to this configuration, the vibrations of the electric motor and an axial stretch thereof are absorbed by the single elastic member.
In the electric power steering apparatus according to the first invention, preferably the steering gear case and the motor housing may be disposed substantially horizontally, and an air gap may be formed between the steering gear case and the motor housing. With this configuration, the dusts drop down from an air gap formed between the steering gear case and the motor housing due to the vibrations etc when traveling.
It is an object of the second invention, which was devised to obviate the problem (2) given above, to provide an electric power steering apparatus capable of ensuring a minimum ground clearance in a vehicle in a way that prevents thermal damages to the electric motor.
To accomplish the above object, an electric power steering apparatus of a rack assist type according to the second invention comprises a steering gear case encasing a power assist mechanism, and an electric motor disposed on the axis different from the steering gear case and used for driving the power assist mechanism, wherein minimum road clearances up to the steering gear case and the electric motor when mounted in a vehicle are made substantially coincident.
According to the second invention, it is possible to increase a distance between the electric motor and a thermal source of an engine etc without decreasing the minimum ground clearance in the vehicle.
In the electric power steering apparatus according to the second invention, preferably the electric motor may be disposed in front of the steering gear case. With this arrangement, a traveling wind is blown directly on the electric motor without being hindered by the steering gear case, thereby restraining a heat emission of the electric motor.
It is an object of the third invention, which was devised to obviate the problem (3) given above, to provide an electric power steering apparatus capable of protecting an electric motor from the splashed water and rebounding pebbles when traveling.
To accomplish the above object, an electric power steering apparatus of a rack assist type according to the third invention comprises a steering gear case encasing a power assist mechanism, an electric motor disposed on the axis different from the steering gear case and used for driving the power assist mechanism, and a power transmission mechanism interposed between the electric motor and the power assist mechanism, wherein a protection cover for covering the electric motor is provided.
In the electric power steering apparatus according to the third invention, preferably a housing accommodating the power transmission mechanism may be covered by a protection cover integral with or separate from the protection cover. With this configuration, the protection cover prevents the water from splashing over the housing for the electric motor and the power transmission mechanism and the rebounding pebbles from impinging thereon.